In addition to their traditional propulsion functions, gas turbine engines are often used as auxiliary power units (APUs) to supply mechanical, electrical, and/or pneumatic power to a wide variety of aircraft systems. For example, the APU can be used to start the main engines, supply compressed air to the aircraft's environmental control system, or provide electrical power. Historically, APU's have only been operated when the aircraft was on the ground.
Recent developments in aircraft design has witnessed the advent of twin engine aircraft capable of long distant, transoceanic flights. Examples of such aircraft are Boeing's 757, 767 and 777, currently under development, as well as Airbus' A300, A310, and A320. A disadvantage to the twin engine design is that when a main engine experiences an inflight shutdown the enormous burden of supplying the aircraft with power falls on the sole, remaining engine. Early on in the development of these aircraft, it was recognized that they would need an additional source of power while inflight. To meet this need it was proposed to start and operate the APU inflight.
A gas turbine auxiliary power unit (APU) includes in flow series arrangement a compressor, a combustor, a turbine, and a shaft coupling the turbine to the compressor. During a normal, sea level start, a starter motor applies a starting torque to the APU's shaft. As the shaft starts to rotate, air is inducted into the compressor, compressed and then discharged in the combustor. Concurrently, the APU's fuel control system feeds fuel into the combustor in accordance with a preprogrammed fuel schedule to precisely maintain the proper fuel to air ratio in the combustor. At a rotational speed of about 10 to 20 percent of the APU's operating speed, the condition in the combustor becomes such that the fuel/air mixture can be ignited. This condition is generally referred to as light-off. Should the fuel to air ratio be either too rich or too lean, light-off will not occur and the APU will experience a hung start. After light-off the start motor torque is augmented by torque from the APU's turbine. At about 50 percent of operating speed the start motor is shut off and the APU becomes self sustaining and accelerates itself to operating speed.
To start an APU at high altitude (e.g. 40,000 ft.) after the APU has become cold soaked by continuous exposure to cold ambient temperatures (e.g. -70.degree. F.) is a much more difficult task for the APU's fuel control system. The cold temperature increases the APU's drag necessitating greater starting torque. Further, cold fuel poorly atomizes. Poor atomization combined with low air density makes it both difficult to precisely obtain the necessary fuel to air ratio to accomplish light-off, and to provide a sufficient fuel flow rate to the combustor to prevent flameout while not providing too high a fuel flow rate which may result excessive turbine inlet temperatures.
The fuel control system used on these APUs has a fuel control unit (FCU) which is typically an electromechanical fuel metering valve disposed between a fuel source and the combustor for feeding fuel to the combustor in response to signals from an electronic control unit (ECU). The ECU contains logic which generates a startup, fuel flow rate signal as a function of shaft speed, inlet pressure and temperature, and engine exhaust temperature (EGT). Included in this logic is a preprogrammed fuel flow rate schedule that specifies the fuel flow rate before lightoff. To prevent flameout or high turbine inlet temperatures after lightoff, this logic usually contains maximum and minimum fuel flow rate limits. The schedule and limits are determined in the laboratory during the development of the APU. During a startup, the ECU monitors shaft speed and EGT and sends the appropriate signals to the metering valve to maintain these parameters within set limits. Thus, in theory these fuel control systems are closed loop systems. An example of such a system is disclosed in Schuh, U.S. Pat. No. 4,627,234.
One deficiency in these prior art fuel control systems, which becomes evident when attempting a high altitude start, is that at low shaft speed and low EGT the instrumentation cannot respond quickly enough or predictably enough to detect and provide a useable signal to the ECU. So in fact at the critical stage of operation just prior to, and after lightoff, the fuel control system is really operating as an open loop system and relying entirely on its preprogrammed fuel schedule.
Another deficiency in these fuel control systems is caused by variations in the performance of different fuel control units. It is well known to those skilled in the art that some FCU's are high side FCU and others are low side FCU. A high side FCU is one that for a given input from the ECU delivers fuel faster than the programmed schedule dictates, and a low side FCU is one that for the same input delivers fuel slower than what the schedule dictates. The problem arises because the ECU also has preset maximum and minimum fuel flow limits for startup. The maximum limit protects against excessive temperatures and surge in the engine and the lower limit protects against flameout and poor fuel atomization. For a high side FCU the limits must be kept low enough to prevent high temperatures, however, for a low side FCU these limits must be kept high enough to prevent flameout. Thus, problems can arise in the field when one FCU replaces another. Importantly, during an emergency start at high altitudes having a mismatch between the FCU and the preset limits could prevent the engine from starting or could considerably damage the engine.
Accordingly, a need exists for a closed loop fuel control system that does not depend on either shaft speed or EGT to adjust the fuel flow rate to a turbine engine during startup, and whose performance is not affected by unit-to-unit variations in FCUs.